1. Field of the Invention
The present invention relates to a line interruption supervisory device for a fire alarm system.
2. Description of the Prior Art
Japanese Utility Model Publication No. 57-38777 describes an example in which line interruption supervision is carried out by the discharge of a terminating capacitor provided across a terminal of lines (electric lines) to which fire detectors are connected.
This example carries out the line interruption supervision by comparing the divided voltage of the power supply voltage with the discharge voltage of the terminating capacitor, thus preventing false operation of the line interruption detecting circuit due to fluctuation of the power supply voltage. If there is no line interruption, the discharge voltage is higher than the divided voltage, and therefore the line interruption indicator does not light up. On the contrary, if there is an open line, the discharge voltage falls below the divided voltage, causing the line interruption indicator to light up. Furthermore, this example is capable of carrying out normal line interruption supervision even if the power supply voltage fluctuates because the divided voltage of the power supply voltage also fluctuates accordingly.
Voltage on lines having fire detectors connected thereto varies with the length of the lines and with the kind and number of fire detectors connected. In the above example, the longer the line length extends and/or the greater the number of fire detectors connected, the lower the discharge voltage becomes. Therefore, the discharge voltage could fall below the divided voltage, causing the line interruption indicator to light up even if there is no line interruption.
The present invention aims at offering a line interruption supervisory device for a fire alarm system which is capable of connected reliably carrying out the line supervision even if the line length is long and/or there is a large number of fire detectors.